Caller ID (CID) is a service which provides the identification of the subscriber line from which a telephone call originates. CID information typically includes the telephone number and area code of the calling party and the time and date of the call. Other information, such as a name associated with the identified telephone number may also be supplied.
There are two different conditions during which caller ID information can be sent to a telephone. These are designated as Type I and Type II Caller ID. In Type I, the dialed telephone is not in use and is in the on-hook state. The CID data is sent as an encoded signal between the first and second rings. In Type II Caller ID, also known as Caller ID with Call Waiting, the dialed telephone is in use and normal voice conversation is interrupted to send the CID data.
FIG. 1 is a block diagram illustrating the relationship between the called and calling telephones, also known as near end and far end customer premises equipment (CPE), and a third-party caller. A telephone call is initiated at the far end CPE. The call is processed by the telephone company's Stored Program Control Switching System (SPCS) and connected to the near end CPE. If the SPCS is configured to provide Type I Caller ID services, the Caller ID data about the far end CPE will be sent by the SPCS to the near end CPE during the interval between the first and second rings.
Once a connection is established between the far end and near end CPE, a Type II Caller ID service providing CID data about third party callers can be provided in conjunction with Call Waiting. The sequence of steps involved in a Type II Caller ID transaction is illustrated in FIG. 2. When a third party call to the near end CPE is detected by the SPCS, audio from the far end CPE is muted and the SPCS sends the near end CPE a Subscriber Alerting Signal (SAS), an audio cue for the listener that indicates a call waiting. Following the SAS signal, the SPCS generates a CPE Alert Signal (CAS) to be used by the near end CPE control circuitry. The CAS signal is a dual tone signal nominally at 2130 Hz and 2750 Hz and is approximately 80 ms in length.
Once the CAS detector in the near end CPE detects the CAS, circuitry in the near end CPE mutes the microphone input signal and generates a dual tone Acknowledge signal (ACK) with a DTMF generator. After the ACK is detected by the SPCS, encoded Caller ID data is sent using frequency shift keyed (FSK) modem tones. Once the FSK data is sent, the SPCS unmutes the far end CPE and the near end CPE microphone is unmuted by its own control circuitry.
In a conventional CPE, the CAS detector and DTMF generator are incorporated into a digital signal processor (DSP). The output signal from the CAS detector is used by the DSP to initiate reception of CID. The DSP typically controls a variety of other telephone features besides Caller ID and contains control circuitry, program and data storage areas, input and output ports, as well as other modules related to audio signal processing.
A major obstacle to designing a reliable CAS detector is the fact that the CAS signal must be detected while speech occurs at the near end CPE. Human speech contains the same audio spectrum as a valid CAS tone. A reliable CAS detector must therefore be sensitive to parameters including the frequency of the two tones in the CAS, the twist of the signal (the strength of one tone relative to the other), the overall level of the CAS signal, and the duration of the signal.
Reliability of a CAS detector is indicated as talkdown and talkoff error rates. A talkdown error is a missed CAS signal in the presence of speech. Talkdown provides a measure of how well the CAS detector is able to detect the alerting signal even with the presence of voice. In normal operation, a talkdown error results only from the near end speech since the far end speech is muted by the SPCS during the near end CPE alerting sequence. A talkoff error results when the CAS detector detects a false CAS from normal speech. This is a serious problem since the false CAS will cause the CPE to mute the transmit path of the microphone and interrupt the conversation. It is critical that the CPE have a robust detector that is not easily triggered by speech. Therefore, the CPE's CAS detector must be rigorously tested to ensure that it operates within acceptable parameters. One procedure for testing a CPE's CAS detector is outlined in the Bellcore document SR-TSV-002476.
Conventional methods of testing a CAS detector involve sending speech alone to the CPE and counting the number of false detects to determine the talkoff error rate. The talkdown error rate is determined by generating a test CAS tone, mixing it with speech to generate an audio test signal which is sent to the CPE, and then counting the number of missed detects. Testing of this type is very time consuming, often taking from 400 to 1,200 hours for a complete test run. Thus, it is advantageous for the talkoff and talkdown tests to be run concurrently.
A significant limitation in conventional testing methods is due to reliance on the dual tone ACK signal to assess the operation of the CAS detector. The ACK signal can be compromised both by problems in the CPE and in the DTMF detector of the testing platform itself. If an ACK signal is missed, an otherwise successful CAS detection may not be recorded. Similarly, erroneous ACK detections may be counted as false CAS detections by the CPE. The results of conventional testing methods are therefore colored by errors introduced by the talkoff and talkdown performance of the DTMF detector in the testing platform.